1. Field of the Invention
The present invention relates to a method and a system for controlling a spark-ignition engine which comprises an abnormal combustion detection device for detecting a preignition which is a phenomenon that an air-fuel mixture self-ignites before a spark-ignited normal combustion initiation timing, an injector for directly injecting fuel into a cylinder, and a variable mechanism for variably setting an intake valve opening timing.
2. Description of the Background Art
Heretofore, in an in-cylinder direct injection engine comprising an injector for directly injecting fuel into a cylinder (combustion chamber) and a spark plug for spark-igniting an air-fuel mixture in the cylinder, a technique has been implemented which is designed to detect whether abnormal combustion where an air-fuel mixture self-ignites before a spark-ignited normal combustion initiation timing (hereinafter referred to as “preignition”) occurs, and, when the preignition is detected, to retard a fuel injection timing from the injector, as disclosed, for example, in JP 2002-339780A (hereinafter referred to as “Patent Document 1”).
More specifically, in the Patent Document 1, in a normal state, a fuel injection timing is set in an intake stroke, whereas, when the preignition is detected, the fuel injection timing is retarded to inject fuel in a compression stroke so as to shorten a period of time between the fuel injection timing and a spark timing (spark ignition timing) (i.e., shorten a heat-receiving period of fuel) to suppress the occurrence of the preignition.
As a method for suppressing the preignition, there has also been known a technique disclosed in JP 2001-159348A (hereinafter referred to as “Patent Document 2”). In the technique disclosed in the Patent Document 2, when an engine speed is equal to or less than a predetermined value, and an amount of change of a required torque in an increasing direction is equal to or less than a predetermined value, it is determined that an engine is under a condition that the preignition is likely to occur. Then, under this condition, an intake valve closing timing is shifted to reduce the effective compression ratio so as to lower an in-cylinder temperature/pressure to suppress the occurrence of the preignition.
The preignition can be suppressed to prevent the occurrence of combustion noise and engine vibration by employing one of the techniques in the Patent Documents 1 and 2. However, when the fuel injection timing is retarded to the combustion stroke, as in the Patent Document 1, an spark ignition operation is performed in a situation where vaporization/atomization of injected fuel is still insufficient, so that a large amount of unburned carbon remains in exhaust gas to produce smoke, which causes a problem of deterioration in emission performance.
On the other hand, when the intake valve closing timing is shifted to reduce the effective compression ratio, as in the Patent Document 2, there is no concern about the occurrence of smoke. However, it involves a problem of lowering in engine power output.